05000498/LER-2014-001
| South Texas Unit 1 2014-001-00 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982014001R00 - NRC Website | |
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I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as a Condition prohibited by Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
Unit 1 was operating in Mode 1 at 100% indicated power, (101.8% actual).
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE AT THE
START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
The Unit 1 overpower value of approximately 1.8% was above the Trip Setpoint Total Allowable value for the Power Range Nuclear Instruments (PRNI) High Trip. The Unit 1 overpower value exceeded the PRNI High Total Allowable value by 0.1%. This exceedence has been attributed to gradual pipe erosion which has taken place since the UTF was commissioned in 1999. Thus, this instrumentation was inoperable longer than the associated allowed outage time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
A reportability evaluation was performed that recommended that this issue be considered REPORTABLE against 50.73(a)(2)(i)(B) — as "Any operation or condition which was prohibited by the plant's Technical Specifications", i.e. Reactor Trip PRNI High Set-points Exceeding the Total Allowable Value of TS 2.2.1, and the associated allowable outage time of LCO 3.3.1.
D. NARRATIVE SUMMARY OF THE EVENT
The Unit 1 Correction Factors for the UTF had been trending in the non-conservative direction since initial installation. After measuring the feed water piping, it was identified that feed water pipe internal diameter measurements were outside the allowances of the UTF calculation. Preliminary analysis indicates that STP Unit 1 exceeded operating limits for thermal output for at least 2.5 years. This condition occurred because pipe internal diameters were not periodically monitored, and the results of the monitoring were not used as a basis to update the calculation.
This condition also occurred in STP Unit 2. This is not reportable in Unit 2 since all the technical specification requirements were met and did not exceed the system set-point allowable values.
E. METHOD OF DISCOVERY
After measuring the feed water piping in December 2013, it was identified that feed water pipe internal diameter measurements were outside the allowances of the respective UTF calculation. Preliminary analysis indicates that STP Unit 1 exceeded operating limits for thermal output for at least 2.5 years.
South Texas Unit 1 05000498 II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
There were no safety systems that responded to this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
One of the PRNI instruments was inoperable for the past 2.5 years. This exceeded the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> TS associated allowable outage time.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
An assessment of the safety consequences and implications is as follows:
Nuclear Safety: Accident Analyses were reviewed to ensure a 2% overpower condition did not consume the Nuclear Safety Margin, and the review concluded that margin remains. Therefore, the overpower conditions of approximately 1.8% in Unit 1 does not result in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety.
The above information is reflected in the PRA in the following way:
- Initiating Events There is no impact on the frequency of initiating events. However, the overpower condition may effect some reactor trip set points such as Power Range, Neutron Flux High and Low, Over-temperature AT, and Overpower AT were set at a higher power level than assumed in the safety analysis.
- Mitigating Systems There is no effect on the PRA mitigating systems directly, but because the plant operated at a higher power there was an associated increase in the decay heat. This small increase in decay heat is not significant, and does not impact the function of these systems.
- PRA Success Criteria and Human Actions The identified overpower condition has a potential impact on PRA success criteria and operator action timing. The impact to these items is not significant because the overpower condition is very small.
Therefore any increase in core damage or large early release risk due to this condition is considered very small.
Security Safety: This condition has no effect on the physical security program at STP. There is no interface between the described condition and security's execution of processes and procedures which implement the requirements of 10 CFR 73.55.
Emergency Preparedness: The described problem has no impact on emergency preparedness equipment and/or systems and on their capability to perform the various required functions.
Radiological Safety: The event did not affect and could not affect the radiological safety of workers. The event was determined to be a level 5 reactivity event and does not affect radioactive process fluids, sources of radiation, safe shutdown of the reactor or an EECS function. Therefore, the event could not cause or increase the likelihood or severity of unintentional exposure to workers.
Safety of station personnel: The event did not affect and could not affect the radiological safety of workers. The event was determined to be a level 5 reactivity event and does not affect radioactive process fluids, sources of radiation, safe shutdown of the reactor or an ECCS function. Therefore, the event could not cause or increase the likelihood or severity of unintentional exposure to workers.
Safety of the public: The event did not affect and could not affect the radiological safety of public, nor could it cause the release of radioactive material. This level 5 reactivity event does not affect radioactive process fluids, sources of radiation, safe shutdown of the reactor or an ECCS function. Therefore, the event could not cause, nor increase the likelihood or severity, of unintentional exposure to the public.
A risk assessment was performed for this event. There was no initiating event or impact to the capability of mitigating systems or other plant equipment to perform required safety functions. Therefore, there is no incremental core damage or large early release risk associated with this event.
Operability/functionality of equipment: The decrease in feed water pipe wall thickness resulted in an increased diameter and corresponding flow area which, in the absence of any other documentation, should have resulted in a higher mass flow rate and corresponding increased secondary calorimetric indication of power. Since the diameter is a constant in the CROSSFLOW computer, the CROSSFLOW computer "under reported" power. The "under reported" power led to calibrating the PRNI, the Overtemperature Delta-Temperature (OTAT), and the Overpower Delta-Temperature (OTAT) trip settings non-conservatively. The diameter change led to the conclusion that overpower was approximately 1.8% for Unit 1.
III. CAUSE OF THE EVENT
The cause of the event has been attributed to an absence of prescriptive procedural requirements for a formal technical review of license changes. STP License Amendment Request implementation process does not require a formal, documented review of plant specific requirements and recommendations from vendor supplied documents that are used as a basis for a license amendment.
IV. CORRECTIVE ACTIONS
Once STP was aware of a potential problem, the vendor, Advance Measurement and Analysis Group (AMAG) was contracted to conduct pipe wall measurements on the Feedwater pipe in the vicinity of the UTF.
This action was performed in December 2013, and was to validate STP pipe inner diameter measurements.
The pipe internal diameter measurements performed by AMAG confirmed that pipe internal diameters were outside uncertainty assumptions. The UTF system was declared unreliable, removed from service, and both units were reduced in power to 3838 mWt. Design changes were issued for the affected units, and the new pipe wall diameters were entered. Unit 1 was returned to full power on 1/17/14.
The following Corrective Action to Prevent Recurrence has been put in place:
Develop or revise a Licensing procedure to require a formal, documented review of plant specific requirements and recommendations from vendor supplied documents to support license amendments.
Corrective actions will be implemented in accordance with the STP Corrective Action Program.
V. PREVIOUS SIMILAR EVENTS
There have been no similar reportable events at STP within the last three years related to this issue.
VI. ADDITIONAL INFORMATION
A supplemental LER report is planned to be submitted to provide additional information regarding the overpower event. A more detail study is being performed by two independent vendors to provide a more detailed analysis of the overpower condition. This submission date of the supplement is projected to be April 30, 2014.